The invention relates to the field of microelectronics, and more particularly, to the sector of fabricating microcomponents, especially those intended to be used in radio or microwave applications. More particularly, it relates to microcomponents of the variable capacitance type made using a technology known by the abbreviation MEMS (Micro-Electro-Mechanical-Systems). This technology makes it possible for conducting levels to be displaced one with respect to the others, due to the effect of a force which may be electrostatic, thermal or magnetic in origin.
The invention is therefore aimed at microcomponents produced using this technology, and which have a capacitance value which can vary over much wider ranges than the existing components.
It is already known to produce capacitive components integrated into microcomponents, using the aforementioned MEMS technology.
Thus, such microcapacitors may have conducting plates, one facing the other, each forming one plate of the capacitor.
Due to the effect of an electrostatic voltage applied between the plates, it is possible to generate a displacement of one plate with respect to the other, and therefore a variation in the capacitance. Such capacitors, known by the generic term of Varicap MEMS, are for example described in the document xe2x80x9cA micromachined variable capacitor for monolithic low noise VCOsxe2x80x9d, D. J. Young and B. E. Boser, Technical Digest Solid State Sensor and Actuator Workshop, pp. 86-89.
The document xe2x80x9cHigh tuning ratio MEMS-based tunable capacitors for RF communications applicationsxe2x80x9d, J. Yao, S. Park, J. DeNatale, Tech. Digest., Solid State Sensor and Actuator Workshop pp. 124-127, 1998 also describes variable capacitors whose plates can be moved one with respect to the other in a direction parallel to the plane of the plates.
The document xe2x80x9cTwo-phase actuators: stable zipping devices without fabrication of curved structuresxe2x80x9d, J. R. Gilbert, S. D. Senturia, Tech. Digest., Solid State Sensor and Actuator Workshop pp. 98-100, 1996 also describes one of the variable capacitors whose two plates form an angle between each other.
Due to the effect of an external force, the angle between the two plates can be altered, and consequently so too can the capacitance of the capacitor formed by these two plates.
These various solutions all have the major drawback of allowing the value of the capacitance to be varied only over a limited range.
Thus, in certain applications for which it is necessary to vary the value of the capacitance considerably, for example in an oscillating circuit whose tuning frequency it is desired to adjust, such components are considered unsuitable.
A first problem which the invention proposes to solve is that of the small variation in capacitance of capacitors made from MEMS technologies.
Moreover, other types of variable capacitors are known, such as, in particular, those described in document xe2x80x9cRF MEMS tunable capacitors for tunable filtersxe2x80x9d, C. L. Goldsmith, A. Malczewski, Z. J. Yao, S. Chen, J. Ehmke, D. Hinzel, International Journal on RF and Microwave Computer-Aided Engineering, 1999, pp. 362-374.
Such a capacitor is made by combining a plurality of elementary capacitors, each one having a predetermined capacitance. These various capacitors are combined by a set of microswitches allowing their parallel placement.
These microswitches are produced in MEMS technology, and are controlled dynamically in order that a certain number of elementary capacitors are placed in parallel.
An overall capacitor of this sort has certain drawbacks. Firstly, the capacitance value of the overall capacitor can only be adjusted to an accuracy corresponding to the capacitance value of the smallest of the elementary capacitors. This adjustment of the capacitance value is therefore not continuous, but discrete.
Furthermore, such a capacitor has a relatively high resistance, since the resistance is equivalent to the resistances of the elementary capacitors produced using traditional technology, increased by that of the microswitches.
Such a resistance is an obstacle to obtaining satisfactory performance, especially in radiofrequency circuits.
A second problem which the invention proposes to solve is that of allowing continuous adjustment of the value of a variable capacitor, as accurately as possible.
The invention relates to a microcomponent including a capacitive component. This microcomponent is characterized in that the capacitive component consists of at least two elementary capacitors connected in series, each elementary capacitor comprising two plates, namely:
a plate fixed with respect to the rest of the microcomponent;
a second plate, part of which is capable of being displaced with respect to the first fixed plate due to the effect of a control signal, so as to vary the value of the capacitance of the elementary capacitor.
In other words, the overall capacitance of the capacitor, the inverse of which is equal to the sum of the inverses of the capacitance of each elementary capacitor, may be adjusted by varying the capacitance of each elementary capacitor.
In a preferred embodiment, it may be possible to displace the movable plate f an elementary capacitor such that this capacitance is particularly high and therefore relatively negligible in the calculation of the overall capacitance. More specifically, the operating mode may be such that all the capacitances except for one are controlled such that the plates are particularly close, and that the capacitance of each elementary capacitor is extremely high.
In this case, the overall capacitance is substantially equal to that of the single capacitor which is controlled differently.
Thus, when the various elementary capacitors have ranges for varying their capacitance which are different, it is possible to provide the overall capacitor with the desired capacitance by choosing the elementary capacitor whose range covers the desired value, and to control the other elementary capacitors such that their capacitance is very high, therefore, negligible.
By choosing variation ranges with a small overlap, the variation range of the overall capacitance is thus optimized.
The architectures used for the elementary capacitors may in particular be varied. Thus, the movable plates can be displaced with respect to the fixed plate in directions perpendicular or parallel to the main plane of the fixed plate.
In another type of elementary capacitor, it may be possible to displace the second plate with respect to the fixed plate by pivoting it about an axis parallel to the latter.
Several technologies may be employed to produce elementary capacitors, and especially technologies using electrolytic deposition of copper.
In a particular embodiment, the elementary capacitors are produced such that the moveable plate forms an arch straddling the fixed plate, with the arch having a deformable central part capable of moving toward the fixed plate due to the effect of the control signal.
In other words, the movable plate forms a bridge which straddles the track forming a fixed plate. The surfaces facing this bridge and the fixed plate form the elementary capacitor. Because of the flexibility of this bridge, it is possible to vary the distance separating the fixed plate and the bridge, such that the capacitance of this elementary capacitor varies.
The arch can be deformed in different ways, for example by applying a continuous bias voltage between the fixed plate and the movable plate.
In practice, the deformation of the bridge makes it possible to vary the capacitance of the elementary capacitor between a high value and a low value.
In practice, the geometry of each elementary capacitor is such that the ratio of these extreme values of capacitance is greater than two, and preferably than five. Under these conditions, the maximum capacitance is relatively large, and it is negligible enough compared to the capacitances of the other elementary capacitors to have a minimal effect on the calculation of the overall capacitance.
In a preferred embodiment, at least one elementary capacitor has a moveable plate which is capable of coming into contact with th fixed plate, and thus of operating as a switch with a capacitance comparable to infinity, without affecting the other elementary capacitors.